The nephron is the filtering unit of the kidney and is essential for regulating blood urea concentration and limiting water and electrolyte loss. Nephron formation is limited to the fetal period in humans and continues to postnatal day 4 (P4) in rodents. After this period of kidney development, new nephrons can no longer be formed. Since the mature kidney lacks an identifiable population of stem cells and has a limited capacity to repair itself after injury, its long-term function relies on nephron over-capacity, which is determined during the fetal/postnatal period (Humphreys et al., 2008; Little and Bertram, 2009). Urea excretion can be augmented by dialysis, but transplantation is eventually required for patients with severe organ function impairment. End-stage renal disease affects approximately 500,000 individuals in the United States and organ availability does not match demand (Abdel-Kader et al., 2009). Technology for ex vivo nephrogenesis would enable therapeutic replacement of damaged kidney tissue, and provide human tissue with which to study kidney development and the origins of kidney disease. Rapid advances in reprogramming somatic cells to the pluripotent state and differentiating these cells through the intermediate mesoderm lineage to nephron progenitors have brought the prospect of generating patient-specific human kidney tissue within reach (Lam et al., 2013; Mae et al., 2013; Taguchi et al., 2014; Takahashi and Yamanaka, 2006; Takasato et al., 2014). While these proof-of-principle experiments have elegantly shown differentiation of nephron progenitors, the numbers of cells that they generate have been relatively modest and identification of procedures to expand these progenitors is still required for practical applications such as engraftment (Lam et al., 2013; Takasato et al., 2014).
The mammalian kidney develops by radial addition of new nephrons that form at the outermost cortex within a progenitor cell niche known as the nephrogenic zone. As the collecting duct branches, progenitor cell aggregates at the collecting duct tips known as cap mesenchyme are induced to differentiate into renal vesicles, polarized derivatives that are the earliest precursors of the epithelial components of the nephron (Mori et al., 2003). The continuous epithelial induction of nephron progenitor cells causes their depletion, necessitating a mechanism to balance progenitor cell renewal with epithelial differentiation, thus enabling multiple rounds of nephrogenesis. Focus on this question over the past 10 years led to the discovery of distinct cell phenotypes, or compartments, that comprise the cap mesenchyme and the specific signaling pathways on which these cells depend (FIG. 6A; Brown et al., 2013; Kobayashi et al., 2008; Mugford et al., 2009; Park et al., 2012).
The least differentiated nephron progenitor compartment is marked by the transcriptional coactivator CITED1 and transcription factor SIX2 (Boyle et al., 2008a; Self et al., 2006). Previous studies have identified essential functions of the BMP, FGF and WNT signaling pathways in regulating the balance between renewal and differentiation in these cells (Barak et al., 2012; Blank et al., 2009; Brown et al., 2011a; Brown et al., 2013; Carroll et al., 2005; Karner et al., 2011).